As a new or reactivating diver, it makes sense to try a few different BC systems before you commit. Returning to diving last year, I learned belatedly about BP/W and now I am all in.
A reg is a reg is a reg. Get a good set. Or two.
On the other side, by 20 dives the equipment could be already paid off. But, of course, trying different things make sense. But trying rentals might still not be a good comparison to stuff that you would end in buying.Don't buy either before your first trip or ~10 dives after OW (at lest 5 of which aren't part of a class). Not a bad idea to wait till > ~20 dives. Buy the one where the variability in rentals bothered you most first.
I'm thinking, find good prices and if it's not something I end up liking, just resell it at breakeven or minimal loss (which would still be less that the rental expense)